A variety of medical instruments are used for the diagnosis and treatment of medical ailments. Transmission of microorganisms to a patient from a medical device can result in a serious disease or death. The medical instrument used to treat the medical ailment is preferably free of microorganisms which thereby minimizes the spread of disease or infection to the patient. A method of sterilizing medical instruments comprises providing an oxidative sterilant or disinfectant in the form of liquid, gas, or gas-plasma to a sealed chamber housing the medical instruments. One sealed chamber type device is the STERRAD.RTM. Sterilization System available through Advanced Sterilization Products of Irvine, Calif., a division of Johnson & Johnson Medical, Inc. Providing the sterilant in gas or plasma form (hereinafter "gas") is especially desirable because the gas renders the surface of the medical device sterile, thereby rendering viruses and bacteria harmless. Likewise, the gas spreads to enclosed or isolated areas of the medical instrument which would otherwise not be sterilized. After a period of time the sterilant in gas form render the devices sterile and a medical technician removes the medical instruments from the chamber.
The importance of achieving complete sterilization motivates placing an indicator in the chamber with the medical instruments to provide verification that an adequate amount of sterilant was provided to the chamber. The prior art method of verifying if the sterilant had entered the chamber comprised placing a chemical indicator and a biological indicator in the chamber. A chemical indicator comprises a surface having a chemical thereon which changes color upon exposure to a sterilization process. Chemical indicators are often integrated with other sterilization verification devices to provide additional evidence of exposure to sterilant. Biological indicators are packages which contain a high number of bacterial spores. The biological indicator, after being exposed to sterilant, is culture tested to determine if the bacteria are viable. If the sterilization was successful, the bacteria contained within the biological indicator will not grow.
In general, chemical indicators should satisfy several basic performance characteristics. The chemical indicator should be readable, reliable, selective, stable, and safe. These performance characteristic are explained in more detail in Volume 1 of the Sterilization Standards Committee of the Association for the Advancement of Medical Instrumentation (AAMI) and by the General Requirements for Chemical Indicators of the proposed American National Standard Institute (ANSI) drafted by Chemical Indicators Working Group. These two documents are incorporated herein by reference.
Chemical indicators of the prior art generally comprise a pH-sensitive material placed on a medium. Simply, the pH-sensitive chemical indicator changes color when exposed to an acidic oxidation-type sterilant, such as H.sub.2 O.sub.2 or H.sub.3 CCOOOH, for a sufficient amount of time. The color change occurs from a chemically induced pH change, i.e., from basic to acid. For example, exposure to an acidic oxidation-type sterilant can change the color of a pH-sensitive chemical indicator from blue to yellow or colorless or from red to yellow. The pH-sensitive chemical indicators of the prior art can also be placed within containers known as test packs. A test pack is a structure which provides a challenge to the sterilization process and thereby provides a more realistic representation of actual conditions of certain areas on medical devices.
The pH-sensitive chemical indicators of the prior art suffer from several disadvantages and may not fully satisfy basic performance characteristics related to stability and selectivity. Since pH-sensitive chemical indicators are chemically reversible, their processed and unprocessed colors can change upon exposure to certain chemicals, especially those with acidic or basic characteristics. Poor pre-processing color stability of pH-sensitive chemical indicators of the prior art is undesirable because it requires chemical indicators to be discarded after a relatively short shelf life thereby wasting supplies. Furthermore, a chemical indicator of the prior art which is on the verge of changing color due to chemical instability does not provide an accurate chemical exposure indication when utilized in a sterilization chamber. Additionally, a short pre-processing shelf life for the chemical indicator is even more undesirable when the chemical indicator is integrated with a biological indicator because an even more expensive device (test pack, including chemical indicator and biological indicator) must be discarded when the pH-sensitive chemical indicator changes color.
Poor post-processing color stability of pH-sensitive chemical indicator of the prior art is also undesirable because the chemical indicator will return to its original color if exposed to a base and, thus, the processed chemical indicator cannot be used as a permanent record of the sterilization process. Therefore, if exposed to a base, the chemical indicator will revert to the original color and provide an unprocessed appearance. This characteristic of pH-sensitive chemical indicators of the prior art is particularly undesirable because using a chemical indicator more than once may provide a faulty processing indication. In a yet different scenario, the indicator could mislead a technician by providing an unprocessed reading when, in fact, the load was processed. This would lead to repeated sterilization cycles thereby increasing cost.
Finally, the pH-sensitive indicators of the prior art are not very selective and may change color upon exposure to any of a number of reagents, not just oxidation-type sterilants. Thus, an indicator may change color from exposure to an acidic reagent and not from adequate exposure to a sterilant. Such an inappropriate color change is misleading and could lead to misinterpretation of result.
Another type of chemical indicator is disclosed in U.S. Pat. No. 5,518,927 entitled "INSTRUMENT STERILIZATION sic! LIFE-SPAN INDICATOR" to Malchesky et al. The Malchesky reference discloses using crystal violet pigment sandwiched between two plastic members and attaching the plastic tag formed therefrom to an instrument during a sterilization process. After repeated exposures, the pigment changes color. Based on the color of the pigment sandwiched between the layers of plastic, the number of sterilizations, i.e. uses, which the instrument has undergone may be determined.
The Malchesky reference discloses a chemical indicator which suffers from numerous disadvantages and drawbacks. First, crystal violet dye possess pH-sensitivity at the extreme lower end of the pH scale. In fact, according to the Handbook of Stains, Dyes, and Indicators by Floyd J. Green, the visual-transition interval is pH 0.0 (yellow) to pH 2.0 (blue-violet) using a .02% aqueous solution of crystal violet. Thus, since crystal violet dye is pH-sensitive, chemical indicators which utilize crystal violet suffer from the same drawbacks as pH-sensitive chemical indicators of the prior art.
In response to these drawbacks, the Malchesky reference teaches enclosing the crystal violet dye in a plastic covering or tag to prevent the acidic sterilant from contacting the pH-sensitive crystal violet dye. However, enclosing the dye in plastic creates further disadvantages. The first disadvantage is that the plastic tag is prone to simply falling off. Alternatively many medical instruments do not provide a place to attach a tag, or if such a place it provided, the tag may severely interfere with the operation and performance of the medical device.
The Malchesky reference also teaches using crystal violet or other organic dyes. However, crystal violet, and many organic dyes in general, are toxic if released from their plastic cover. For example, crystal violet, which has catalog number 229288 in the Sigma-Aldrich Chemical Company database, is a cationic triarylmethane dye, is toxic and may cause cancer, heritable genetic damage and irritate the eyes, respiratory system and skin. Thus, the invention of the Malchesky reference suffers from another serious drawback.
Therefore, a need exists for a chemical indicator for use with oxidation-type sterilization systems which does not chemically break down, has increased sensitivity and will not give false indications upon exposure to common chemical compounds and conforms with the performance characteristic of the AAMI Sterilization Standards Committee. The present invention satisfies that need.